The present invention relates to information transmission network systems complying with standardized methods such as SDH (Synchronous Digital Hierarchy) and SONET (Synchronous Optical Network) and node equipment for use in those network systems, and particularly to a control technology for switching between the service lines and the protection lines.
In recent years, a variety of high speed data transmission services for information transmission network systems have been proposed along with intensive relevant technological developments. For the implementation of high speed transmission services, multiplexing methods have to be standardized so that existing low speed transmission services may also be multiplexed together in the same lines used for high speed services. SONET and SDH are examples of such standardized methods. Note that SONET is America's proprietary technique related to SDH.
In many cases, SONET/SDH-based information transmission network systems (hereafter, SONET/SDH transmission systems) have adopted a redundancy configuration having both service and protection lines for their interface equipment and transmission paths inside the node equipment. Under this redundant configuration, the communication path that has been set in the service line for service traffic can be switched to the protection line in case of failure in the service line. Service reliability is thereby maintained even in the events of failure and breaks in lines.
In some of such redundant SONET/SDH transmission systems, the protection line is vacant during normal operation of the service line. For this reason, the traffic capacity of the whole network is raised by setting a communication path in the protection line in order to accommodate another flow of traffic. This traffic that travels in the communication path in the protection line includes part-time traffic (hereafter, P/T traffic) and the extra traffic defined by the ITU-T Recommendations. Information carried in the P/T traffic is handled as its transmission may be discarded temporarily. This is because the communication path for the service traffic set in the service line can be instantaneously switched to the protection line by releasing the protection line in case of failure in the service line.
This service/protection lines switching function for the case of a system failure is referred to as APS (Automatic Protection Switching) in SDH. The details of APS have been described in the ITU-T Recommendation G.841 (latest revised, October/1998). As described in this recommendation, in case of a failure, each node on the network automatically switches between the service and protection lines by rewriting K1, K2 bytes (hereafter, K-byte) defined in the overhead of each SDH transmission frame and exchanging these rewritten signals to recognize the occurrence of a failure.
During the service/protection lines switching in the event of a problem in the communication path set in the service transmission line, the communication path is switched to the protection line to save the information in the service traffic. At this switching operation, error connections (so called misconnect) of the service traffic should be prevented. In the event of a misconnect, the information carried in the service traffic may be leaked to an undesired party, causing serious data security problems.
Therefore, the ITU-T G.841 Recommendation makes it a rule to insert a predetermined signal called AU-AIS (Administrative Unit-Alarm Indication Signal) at the exit to lower-order groups, which is the exit of the existing P/T traffic path in the protection line, prior to the traffic switching to the protection line. Then the parties connected to this communication path cannot interpret the delivered information and eventually the P/T traffic path is terminated. Namely, since the service traffic communication path is changed after the P/T traffic path has been terminated in SONET/SDH transmission systems, it is ensured that the information in the service traffic is not leaked to the destinations of the P/T traffic path. Note that “AU-AIS” is a name used in the SDH Standards.
On the other hand, the above recommendation has not identified a rule applied to the protection line that does not have a P/T traffic communication path. However, it is possible that the protection line is connected to multiplexes and communication units (hereafter, lower-tier terminals) existing on the lower-tier side in each node, substantially forming communication paths with such lower-tier terminals. Then, if the service traffic communication path is switched to a protection line having no P/T traffic path, a misconnect can be made to a party existing on the substantially established communication path and the information in the service traffic may be leaked to this party.